Impact of dextran sulfate in culture media on titration of vesicular stomatitis virus

Abstract Viral vectors derived from vesicular stomatitis virus (VSV) are important vectors for the development of vaccines and for the treatment of cancer. The efficiency of therapy based on VSV is dependent on the dose of virus used. Therefore it is essential to measure accurately and reproducibly the amount of functional vectors in the samples to be tested. Two common methods used to measure the titer of VSV are TCID50% and plaque assay. In the current study, we compared these two titration methods by using a recombinant VSV expressing the green fluorescent protein (VSV-GFP) as a model virus. Some culture media developed for suspension mammalian cells contain dextran sulfate. We observed that plaque assay, but not TCID50%, can underestimate the virus titer up to 10 fold when VSV-GFP was produced in culture media containing dextran sulfate. Dextran sulfate is commonly used in serum-free culture media to reduce cell aggregation in suspension culture. The inhibitory effect of dextran sulfate on the titration of VSV-GFP was confirmed by supplementing the culture medium with this compound during virus production. Our results also demonstrated that extending the incubation time during plaque assay and TCID50% increases virus titer.